Until fairly recently, the cable network was predominantly a vehicle for delivering entertainment. With the advent of the Internet and the rise in demand for broadband two-way access, the cable industry began to seek new ways of utilizing its existing plant. Pure coaxial (“coax”) cable networks were replaced with hybrid fiber networks (HFNs) using optical fiber from the head end to the demarcation with the subscriber coax (usually at a fiber node). Currently, a content-based network, a non-limiting example of which is a cable television network, may afford access to a variety of services besides television, for example, broadband Internet access, telephone service, and the like.
One significant issue for a cable operator desiring to provide digital service is the configuration of its network. Designed for one-way delivery of broadcast signals, the existing cable network topology was optimized for downstream (toward the subscriber) only service. New equipment had to be added to the network to provide two-way communication. To reduce the cost of this equipment and to simplify the upgrade of the broadcast cable for two-way digital traffic, standards were developed for a variety of new cable-based services. The first of these standards, the Data Over Cable System Interface Standard (DOCSIS® standard), was released in 1998. DOCSIS® establishes standards for cable modems and supporting equipment. DOCSIS® (Data Over Cable Service Interface Specification) is a registered mark of Cable Television Laboratories, Inc., 400 Centennial Parkway Louisville Colo. 80027, USA, and will be referred to for the remainder of this application in capital letters, without the ® symbol, for convenience.
IP addresses are allocated in blocks known as subnets or prefixes on a network. These addresses are regularly allocated and moved as part of network growth and expansion. A cable modem termination system or CMTS is a piece of equipment typically located in a cable company's head end or hub site, and used to provide high speed data services, such as cable Internet or voice over Internet Protocol (VoIP), to cable subscribers. A CMTS provides many of the same functions provided by the digital subscriber line access multiplexer (DSLAM) in a digital subscriber line (DSL) system.
On a DOCSIS network, IP subnets are allocated on a per-CMTS basis.